Low-side bidirectional battery disconnect switch

ABSTRACT

A bidirectional battery disconnect switch, i.e., a switch which is capable of blocking a voltage in either direction when open and conducting a current in either direction when closed, is disclosed. The switch includes a four-terminal MOSFET having no source/body short and circuitry for assuring that the body is shorted to whichever of the source/drain terminals of the MOSFET is biased at a lower voltage.

This application is a continuation of application Ser. No. 08/471,133,filed Jun. 6, 1995, now abandoned which is a division of applicationSer. No. 08/367,515, filed Dec. 30, 1994 now U.S. Pat. No. 5,689,209.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to application Ser. No. 08/160,560, filedNov. 30, 1993, application Ser. No. 08/160,539, filed Nov. 30, 1993, andapplication Ser. No. 08/219,586, filed Mar. 29, 1994, each of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to battery disconnect switches and, inparticular, to battery disconnect switches that are entirelybidirectional, i.e., capable of blocking or conducting a current ineither direction.

BACKGROUND OF THE INVENTION

A battery disconnect switch is a bidirectional switch that is used forenabling or disabling a current flow between a battery and a load or abattery and a battery charger. Since the battery charger may provide avoltage that is greater than the battery voltage, the switch must beable to prevent a current flow from the charger to the battery when theswitch is in an off condition. Conversely, the switch must be able toblock a current flow in the opposite direction when the load has a lowresistance, when the load is shorted, or when a battery charger has anincorrect output voltage or is connected with the polarity of itsterminals reversed.

Moreover, when the switch is turned on, in most situations the switchmust be capable of conducting a current in either direction. When thebattery is charging, a current flows from the battery charger throughthe switch to the battery. During normal operation, a current flowsthrough the switch in the opposite direction from the battery to theload. Accordingly, the purpose of the battery disconnect switch is toprovide a means to enable or interrupt current conduction between abattery and other electrical devices, regardless of the current orvoltage to which the switch is exposed. Current interruption isparticularly valuable to provide the following functions: preventovercharging of a battery; prevent over-discharging of a battery;prevent excessive currents from flowing in the case of a shorted load orbattery; protect a battery from improper battery charger connections(such as a reverse connection); extend battery shelf life by minimizingleakage, which will tend to discharge the battery over extendeddurations; and facilitate sequencing or switching between multiplebatteries and multiple loads.

While conventional power MOSFETs (in which the source and body areshorted) may be used to form a battery disconnect switch, the presenceof a single P-N junction diode between the drain and the source makes asingle power MOSFET incapable of bidirectional current blocking. Sincebidirectional current blocking between two or more power sources is anecessary function of all battery disconnect switches, the use ofdiscrete power MOSFETs requires that two devices be placed back-to-backin series, with either a common source or a common drain. The totalon-resistance of the switch is then twice that of an individual powerMOSFET. Such an arrangement is shown in FIG. 1A, wherein MOSFETs 10 and11 are connected in a common source configuration.

An alternative is to use a symmetrical drifted lateral MOSFET not havinga source/body short. Such an arrangement is described in theabove-referenced application Ser. No. 08/219,586, application Ser. No.08/160,539, and application Ser. No. 08/160,560. An example of this typeof arrangement is shown in FIG. 1B wherein a symmetrical MOSFET 12 hasone terminal connected to the high side of battery 13 and the otherterminal connected to the load. (While MOSFET 12 is symmetrical, theterminal connected to battery 13 is arbitrarily referred to as thedrain, and the terminal connected to the load is arbitrarily referred toas the source.) The body of MOSFET 12 is connected to the negativeterminal of battery 13, which is normally grounded, so that thedrain-to-body diode within MOSFET 12 is reverse biased. The load isconnected with its negative terminal directly connected to ground whileits positive terminal is connected to the source of MOSFET 12. As aresult, whether MOSFET 12 is off or on, its source-to-body diode, likeits drain-to-body diode, remains reverse biased under all normalconditions.

MOSFET 12 offers a low on-resistance while being able to block currentin both directions when it is turned off. Also, since MOSFET 12 isconnected to the high side of battery 13, the grounded low side of thebattery can form a common conductive plane in a printed circuit board.In many situations, this helps reduce noise and makes the wiring of theprinted circuit board relatively straightforward. MOSFET 12 may beturned off easily by grounding its gate. (However, extra circuitry isrequired to protect against a reversed battery charger condition. Anexample of such circuitry is described in the above-referencedapplication Ser. No. 08/219,586. During conduction, the battery voltageincreases the reverse bias on the source-to-body junction of MOSFET 12,leading to the well known “body effect” wherein the threshold voltage ofthe MOSFET increases. Assuming a fixed gate drive voltage, the bodyeffect increases the on-resistance of the device. Also, the gate ofMOSFET 12 must be driven above the battery voltage to guarantee a lowon-resistance. This requires a charge pump to generate a positive supplyabove the voltage of the battery (requiring an oscillator which consumespower).

SUMMARY OF THE INVENTION

In accordance with this invention, a bidirectional battery disconnectswitch (BDS) is connected to the low side of the battery, which istypically grounded. The BDS includes a switch MOSFET which issymmetrical, having no source/body short.

Circuitry is provided to connect the body of the MOSFET to whichever ofthe switch MOSFET's terminals is biased more negatively. In a preferredembodiment, this circuitry includes a pair of MOSFETs. A first MOSFET isconnected between the battery-side terminal of the switch MOSFET(arbitrarily designated the drain) and the body of the switch MOSFET. Asecond MOSFET is connected between the body of the switch MOSFET and theload-side terminal of the switch MOSFET (arbitrarily designated thesource). The gate of the first MOSFET is connected to the source of theswitch MOSFET; the gate of the second MOSFET is connected to the drainof the switch MOSFET. Accordingly, when the drain of the switch MOSFETis at a higher voltage than the source of the switch MOSFET, the secondMOSFET is turned on and shorts the body and source of the switch MOSFET.Conversely, when the source of the switch MOSFET is at a higher voltagethan the drain of the switch MOSFET, the first MOSFET is turned on andshorts the body and drain of the switch MOSFET.

The switch MOSFET is turned on by connecting its gate to the positivebattery voltage and turned off by grounding its gate. In a preferredembodiment, the gate of the switch MOSFET is tied to its body, which isgrounded.

When the switch MOSFET is turned on, the voltage difference between itssource and drain is relatively small, and neither of the first andsecond MOSFETs is turned on. This allows the body of the switch MOSFETto float. If enough current is forced into the body of the switch MOSFETwhile it is turned on, the body potential of the switch MOSFET couldconceivably rise above the potential of both its source and drainterminals, thereby forward-biasing both its drain-to-body andsource-to-body diodes. This could create an excessive source-to-draincurrent in the switch MOSFET and could damage the device.

A second pair of MOSFETs is used to prevent this from happening. Thispair includes a third MOSFET, which is connected in parallel with thefirst MOSFET, and a fourth MOSFET, which is connected in parallel withthe second MOSFET, the respective gates of the third and fourth MOSFETsbeing tied to the body of the switch MOSFET.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a conventional battery disconnect switch whichincludes a pair of MOSFETs connected back-to-back.

FIG. 1B illustrates a prior art battery disconnect switch which includesa single MOSFET not having a conventional source/body short.

FIG. 2 illustrates conceptually a battery disconnect switch inaccordance with this invention.

FIGS. 3A and 3B illustrate ways in which unwanted conduction may occurin a battery disconnect switch.

FIGS. 4A-4H illustrate the required connection of the body of the MOSFETin the battery disconnect switch, for a variety of on and off switchconditions.

FIG. 5 illustrates a conceptual block diagram of a battery disconnectswitch in accordance with this invention, including a body biasgenerator and a current source which represents the body current in thepower MOSFET.

FIG. 6 illustrates a circuit diagram of a battery disconnect switch inaccordance with this invention.

FIG. 7A illustrates the conditions in which the MOSFET linking the bodyand drain of the switch MOSFET will be turned on.

FIG. 7B illustrates the situation in which the MOSFET linking the bodyand source of the switch MOSFET will be turned on.

FIG. 7C illustrates a circuit diagram of an overcurrent detectioncircuit for use in conjunction with the battery disconnect switch.

FIG. 8 illustrates a graph showing the experimentally measured bodycurrent in the switch MOSFET as a function of gate voltage (V_(gs)) forvarious drain voltages (V_(ds))

FIG. 9 illustrates a graph showing the gate width of MOSFETs M2 and M3that is required to sink a specified body current in the switch MOSFETwithout forward-biasing the source-to-body diode in the switch MOSFETmore than 0.3 V.

FIG. 10 illustrates a graph showing the current through the switchMOSFET as a function of the drain-to-source voltage at various gatedrives.

FIG. 11 illustrates a graph showing the current-voltage characteristicsof MOSFETs M2 and M3 as a function of gate drive.

FIG. 12 is a circuit diagram which describes the effect of the bodycurrent in the switch MOSFET.

FIG. 13 illustrates a graph showing the drain current I_(d) in theswitch MOSFET as a function of the source-to-drain voltage V_(ds) fordifferent body currents when MOSFETs M2-M5 are absent and the gate andbody of the switch MOSFET are tied together.

FIG. 14 illustrates a graph showing the drain current I_(d) in theswitch MOSFET as a function of the source-to-drain voltage V_(ds) fordifferent body currents I_(b) when MOSFETs M2 and M3 are present.

FIG. 15 illustrates a graph of the kind shown in FIG. 14 for a bodycurrent I_(body)=32 μA.

FIG. 16 illustrates a circuit diagram of the operation of MOSFETs M4 andM5.

FIG. 17 illustrates a current-voltage diagram which compares thecharacteristics of MOSFETs M4 and M5 with a conventional P-N diode.

FIG. 18 illustrates a diagram which compares the current-voltagecharacteristics of MOSFETs M4 and M5 with a conventional P-N diode andwith a Schottky diode.

FIG. 19 illustrates a graph showing the current-voltage characteristicsof the parallel combinations of MOSFETs M2/M4 and M3/M5 as a function ofthe gate drive on MOSFETs M2 and M3, respectively.

FIG. 20 illustrates a graph showing the current-voltage characteristicsof the battery disconnect switch shown in FIG. 6 with the switch in anoff condition and with a body current I_(body) of 30 μA.

FIG. 21 illustrates a graph showing the same information as FIG. 20 withbody currents of 20, 40, 60 and 80 μA.

FIG. 22 illustrates a graph showing the voltage across MOSFET M4 or M5as a function of gate width at various threshold voltages V_(t) and withthe MOSFET sinking a current of 30 μA.

FIG. 23 illustrates the current-voltage characteristic of thebidirectional switch shown in FIG. 6 for several different levels ofgate drive (V_(gs)).

FIGS. 24A-24C illustrate circuit diagrams showing alternative methods ofpreventing an excessive voltage across the gate oxide of MOSFET M2 or M3when a battery charger is connected in reverse.

FIGS. 25A and 25B illustrate cross-sectional views of the circuits shownin FIGS. 24A and 24C, respectively, in the form of an integratedcircuit.

FIG. 26 illustrates a graph showing the threshold V_(t) of a MOSFET as afunction of the source-to-body voltage V_(sb) at different levels ofbody factor (γ)

FIG. 27 illustrates a block diagram of an alternative embodiment whichcontains two body bias generators.

DESCRIPTION OF THE INVENTION

FIG. 2 illustrates a bidirectional battery disconnect switch S1 inaccordance with this invention. Switch S1 is a “low side” switch in thesense that it is connected to the “low” or negative side of a battery B.A load X is connected to the “high” side or positive terminal of batteryB. Switch S1 includes a switch power MOSFET M1 which is a symmetricaldevice, not having a conventional source-body short. The terminal ofMOSFET M1 which is connected to battery B is referred to as the drainterminal, and the terminal of MOSFET M1 which is connected to the load Xis referred to as the source terminal. These designations are somewhatarbitrary because, switch S1 being bidirectional, either terminal couldbe more positive in a given situation.

Battery disconnect switch S1 also includes a switch S2 connected betweenthe body and drain of MOSFET M1 and a switch S3 connected between thebody and source of MOSFET M1. Switch S2 can be closed and switch S3 canbe open, as shown in FIG. 2 or, alternatively, switch S2 can be open andswitch S3 can be closed. Switches S2 and S3 are never closedsimultaneously because in that event MOSFET M1 would be short circuited.Switches S2 and S3 operate so as to short the body of MOSFET M1 towhichever of its drain and source terminals is at a lower voltage. Thus,as shown in FIG. 2, switch S2 would be closed when the drain of MOSFETM1 is at a lower voltage than its source. Switch S1 is turned on byapplying the voltage at the positive terminal of battery B to the gateof MOSFET M1. Switch S1 is turned off by connecting the gate of MOSFETM1 to the body of MOSFET M1. (Depending on which of switches S2 and S3is closed, the gate is also connected to the source or drain of MOSFETM1, whichever is biased more negatively.) When the gate of MOSFET M1 isconnected to the positive terminal of battery B, switch S1 is turned onand will conduct current in either direction, depending on the relativevoltages at the drain and source of MOSFET M1.

The body and gate voltages of MOSFET M1 should be selected to assurethat MOSFET M1 is turned off whenever this is desired, regardless of thevoltages at the source and drain of MOSFET M1. Unwanted conduction in an“off” switch can lead to overcharging or unavoidable discharging of abattery. The gradual discharging of a battery cell shortens the batterylife. In some battery technologies such as the lithium ion technology,overcharging may actually damage the battery cell permanently.Overcharging may also present a safety hazard.

FIGS. 3A and 3B illustrate two ways in which unwanted conduction mayoccur. In FIG. 3A, the body of MOSFET M1 is connected to its sourceterminal when the source terminal is biased positively with respect tothe drain terminal. In this situation, a current will flow in MOSFET M1despite the fact that the MOSFET is turned off, i.e., its gate is tiedto the more negative drain potential. The diode between the body anddrain of MOSFET M1 is forward-biased and, assuming that the load has aresistance approaching zero, a significant current I_(diode) will flowthrough MOSFET M1.

In FIG. 3B a battery charger Y is connected in the circuit. If batterycharger Y has a voltage which is greater than the battery voltageV_(batt), the source of MOSFET M1 will be negatively biased with respectto its drain. Here the body of MOSFET M1 is shorted to its source butits gate is connected to the more positively biased drain of MOSFET M1.The gate bias V_(gs) of MOSFET M1 is therefore equal to the differencebetween V_(charger) and V_(batt). If the on-resistance of MOSFET M1 islow, even a small voltage mismatch between battery charger Y and batteryB will allow amperes of current to flow through MOSFET M1 even thoughits gate is at ground potential.

Even worse is a combination of improper body and gate connections inMOSFET M1, as a result of which, both a channel current and a diodecurrent may flow through MOSFET M1.

Accordingly, a battery disconnect switch in its off condition mustprevent a channel current while preventing either of its intrinsic P-Nbody diodes from becoming forward-biased. Even in the “on state”, it isdesirable for the intrinsic diodes in the MOSFET to remainreverse-biased. In accordance with this invention, the solution to thisproblem is to provide circuitry which in effect determines whether thesource or the drain of the MOSFET is more negative and then connects thebody of the MOSFET to the more negative potential, thereby shunting theforward-biased diode by another smaller MOSFET. The other diode in theMOSFET then is reverse-biased, and parasitic conduction is avoided.

FIGS. 4A-4H illustrate the required body connection for a variety of onand off switch conditions. FIG. 4A shows the normal condition in whichthe load X is connected to the circuit and the switch S1 is turned on.The voltage across the switch S1 is therefore relatively small.Nonetheless, the battery side (drain) of the switch S1 is at the morenegative potential, and the body must therefore be connected to thedrain of MOSFET M1. The gate of the MOSFET is connected to V_(batt). Inthe comparable “off” situation illustrated in FIG. 4B, essentially theentire battery voltage appears across switch S1. The gate of MOSFET M1is connected to its body, which is shorted to its drain.

FIGS. 4C and 4D illustrate the situation in which the load is shorted.Assuming that switch S1 is turned on when the short occurs, the entirebattery voltage will be present momentarily across MOSFET M1. Since thegate and drain of MOSFET M1 are shorted, MOSFET M1 saturates anddissipates high power levels. If this is left unchecked, heating willquickly destroy the device, assuming that the bond wires can handle thecurrent. If the overcurrent condition is detected by the drive circuitry(see FIG. 7C), the MOSFET can be quickly turned off by shorting its gateto its body, as shown in FIG. 4D. In the off condition, MOSFET M1withstands the voltage V_(batt) across its terminals.

The highest voltages appear across switch S1 when battery charger Y isconnected in reverse. At the onset of the reverse connection, whenswitch S1 is on, a high current results. This situation is illustratedin FIG. 4E and is similar to the situation shown in FIG. 4C. The maximumvoltage present momentarily across switch S1 depends on thecurrent-voltage characteristics of charger Y. The maximum voltage willbe in the range between V_(batt) and 2V_(batt). After the faultcondition is detected and switch S1 is opened, as shown in FIG. 4F, thecurrent through switch S1 drops to zero. In this condition, switch S1encounters the summation of V_(batt) and the voltage across the batterycharger, which may be as high as 2V_(batt). The terminal of M1 connectedto the charger (i.e., the “source” terminal) becomes the more positiveterminal and the “drain” terminal becomes the more negative terminal ofMOSFET M1. The body of MOSFET M1 should therefore be shorted to itsdrain terminal. Provided that proper gate control circuitry is included,that circuitry detects the overcurrent condition and opens switch S1, asin the case of a shorted load (FIG. 4C). The maximum voltage acrossswitch S1 in the shorted load case, however, is only V_(batt), while thevoltage in the case of a reversed battery charger is potentially twicethat value. If the battery charger has a high series resistance, thedischarging current may not trip the overcurrent protection circuitry,in which case the battery charge will be depleted.

FIGS. 4G and 4H show the situation with battery charger Y properlyconnected. As the battery voltage increases during current-limitedcharging, so too does the charger voltage. In this case, the body ofswitch S1 should theoretically be tied to its drain, since the drain ismost negatively biased. In reality, the body connection duringconduction is relatively unimportant because the voltage drop acrossswitch S1 is very small.

When battery charger Y no longer supplies a charging current, as shownin FIG. 4H, either due to overcharging of battery B or some faultcondition, the voltage of battery charger Y will likely increase to avoltage larger than the battery voltage, i.e., V_(charger) is greaterthan V_(batt), Since the battery B and charger Y share a common positiveterminal, the charger Y pushes the drain of switch S1 to a voltage belowthe negative terminal of battery B. That is, the drain of MOSFET M1becomes the most negatively biased point in the circuit. The body ofMOSFET M1 must therefore be connected to the drain to avoid unwantedconduction. The voltage across switch S1 is equal to the differencebetween V_(changer) and V_(batt), At the commencement of charging thisvoltage difference could be as high as 8.0 to 9 V. After charging, thedifference would typically be no greater than 0.5 V, depending on thetype of battery charger.

To summarize the situations illustrated in FIGS. 4A-4H, the onlycondition where the body of MOSFET M1 is not connected to the negativeterminal of battery B (i.e., the “drain” of the MOSFET) is where abattery charger is connected in a correct polarity. In the latter event,the body of MOSFET M1 is connected to the negative terminal of thecharger (i.e., the “source” of the MOSFET). Whenever switch S1 is turnedon, its gate is biased to the positive battery voltage (V_(batt)) andwhen switch S1 is turned off, its gate is tied to its body.

Thus, what is needed to perform this function is a pair of switches thatwill short the body of the MOSFET to either the source or drainterminal, whichever is biased more negatively. In addition, the switchesmust satisfy the following conditions:

1. The switches should never be on at the same time or they will providea path parallel to the power MOSFET which will allow a leakage to flowin the battery disconnect switch in its off state.

2. The switches cannot allow the voltage of the body to float withoutrisking conduction in the MOSFET due to subthreshold channel conductionor forward-biasing of one of its intrinsic diodes. In subthresholdconduction (where the gate of the MOSFET is tied to its body), agate-to-body voltage which turns on the MOSFET develops anytime the gatefloats to a potential more positive than the source. Even at 0.5 V ofenhancement, the large transconductance of a low on-resistance switchcan produce hundreds of milliamps of channel current. Beyond a gatevoltage V_(gs) of 0.6 V, partial forward-biasing of either thedrain-to-body or source-to-body diode will lead to excessive currentswhich may be further amplified by impact ionization or by doubleinjection (wherein the parasitic NPN bipolar transistor present in allis N-channel MOSFETs conducts).

These two requirements are somewhat inconsistent. The first requiresthat one of the switches open before the other closes, and during thisperiod the body will necessarily float. In either case, adrain-to-source current leakage may occur in the MOSFET M1, and this mayallow a battery to become overcharged or over-discharged. What is neededis an arrangement which has an extremely small, but non-zero “deadband”(i.e., the voltage range in which neither switch conducts) at all timesand under all variations in manufacturing.

FIG. 5 illustrates a block diagram of a body bias generator 50 whichconnects the body of MOSFET M1 to its source or drain terminal inaccordance with the above-described conditions. Current source 51represents schematically the current from any integrated circuit or gatebuffer used to control the condition of MOSFET M1 which the bodyterminal of the MOSFET M1 must sink. Terminals T1 and T2 are connectedto a load or battery charger.

FIG. 6 illustrates a circuit diagram of an embodiment according to theinvention. Battery disconnect switch S6 includes switch MOSFET M1 andbody bias generator 50. A terminal T3 is connected to the negativeterminal of a battery and a terminal T4 is connected to a load orbattery charger. Body bias generator 50 includes a first pair of MOSFETsM2 and M3 and a second pair of MOSFETs M4 and M5.

MOSFET M2 is connected between the drain and body of MOSFET M1, andMOSFET M3 is connected between the source and body of MOSFET M1, withthe source terminals of MOSFETs M2 and M3 being connected to the body ofMOSFET M1. MOSFETs M2 and M3 contain a conventional source-body short.The gate of MOSFET M2 is connected to the source of MOSFET M1, and thegate of MOSFET M3 is connected to the drain of MOSFET M1.

MOSFETs M4 and M5 are connected in parallel with MOSFETs M2 and M3,respectively. The gate terminals of MOSFETs M4 and M5, however, areconnected in common to the body of MOSFET M1. The source and bodyterminals of MOSFETs M4 and M5 are shorted in the conventional manner tothe body of MOSFET M1.

MOSFETs M2 and M3 function to short the body of MOSFET M1 to whicheverof the source and drain terminals of MOSFET M1 is at a lower voltage.MOSFETs M4, and M5 function to prevent the body of MOSFET M4 from“floating” upward to an excessive degree when MOSFETs M2 and M3 are bothturned off.

The operation of MOSFETs M2 and M3 will be described first. Because thegate terminals of MOSFETs M2 and M3 are cross-coupled to the source anddrain terminals of MOSFET M1, respectively, MOSFET M2 will turn on (FIG.7A) whenever the voltage at the source exceeds the voltage at the drainof MOSFET M1, and MOSFET M3 will turn on (FIG. 7B) whenever the voltageat the drain exceeds the voltage at the source of MOSFET M1. In otherwords, MOSFET M2 functions to short the drain and body of MOSFET M1 whenthe voltage at the drain is lower than the voltage at the source ofMOSFET M1, and MOSFET M3 functions to short the body and source ofMOSFET M1 when the voltage at the source is lower than the voltage atthe drain of MOSFET M1. Thus, the coordinated operation of MOSFETs M2and M3 satisfy condition (1) above, namely the body of MOSFET M1 isclamped to whichever of the drain and source terminals of MOSFET M1 isbiased most negatively. This assumes, of course, that the gate to sourcevoltage of one of MOSFETs M2 and M3 exceeds the threshold voltagenecessary to turn the MOSFET on. The situation when this is not thecase, i.e., MOSFETs M2 and M3 are both turned off, is discussed below.

When switch S6 is connected to a load (whether normal or shorted) or abattery charger with its terminals reversed, the drain of MOSFET M1 (orterminal T3) is biased most negatively. The maximum voltage that switchS6 would normally have to withstand for a fully charged batteryconnected to a load is 8.4 V. In the reversed battery charger condition,switch S1 must withstand a voltage equal to V_(batt)+V_(charger). Thisvoltage normally does not exceed 16.8 V. With an effective overvoltagedetection circuit, however, current is not expected to flow for voltagesover 8.4 V, and even if it does there should be no problem so long asthe current flows during a switching transient where the total power islimited. Both MOSFET M1 and whichever of MOSFETs M2 and M3 is turned offmust be able to withstand these voltages.

When the battery charger is properly connected, the negative terminal ofthe charger (not the battery) is the most negative point in the system,and MOSFET M3 therefore turns on. When MOSFET M1 is off, the voltageacross switch S6 is equal to V_(changer)−V_(batt). The open circuitvoltage of the battery charger may actually exceed 8.4 V. For a fullydischarged battery, this voltage is present across switch 56. As thebattery charges, the voltage across switch S6 declines. When theovercharge condition of the battery is detected, the maximum voltageacross switch S6 is equal to V_(charger)−8.4 V. None of these voltagesis large enough to create a problem in switch S6, even when current isflowing (assuming that the maximum power is limited by the batterycharger's current compliance circuitry).

There are numerous known techniques of controlling the gate voltage onMOSFET M1. As noted above, for example, a overcurrent detection circuitmay be required to drive the gate of MOSFET M1 low, thereby turningMOSFET M1 off, when the load is shorted. Such an arrangement isillustrated in FIG. 7C. A resistor R delivers a voltage drop whichexceeds a predetermined threshold when an excessive current flowsthrough MOSFET M1. This voltage is detected by a comparator S whichdelivers an output signal through an OR gate T to an input of aninverter U. Inverter U delivers an output which in effect clamps thegate and body of MOSFET M1. As described elsewhere, this turns MOSFET M1off. Absent an overcurrent condition, a gate control signal delivered toanother input of OR gate T controls the gate of MOSFET M1. When the gatecontrol signal is low, inverter U applies V_(batt) to the gate of MOSFETM1, turning MOSFET M1 on; when the gate control signal is high the gateand body of MOSFET M1 are tied together. Comparator S, OR gate T andinverter U may be included in an integrated circuit, and body biasgenerator may prevent the forward-biasing of diodes in the integratedcircuit. Numerous other methods of controlling the gate of MOSFET M1 soas to satisfy various conditions are known in the art.

Aside from holding the body to the most negative potential in the offstate, MOSFETs M2 and M3 must maintain a proper bias of the body ofMOSFET M1 in the presence of any impact ionization. Impact ionizationoccurs in a MOSFET when both current and large voltages aresimultaneously present. Such a condition requires the MOSFET to be inits saturation region of operation. Since MOSFET M1 would typically havea low on-resistance and large channel width, the transconductance of thedevice is very high. Even a modest gate drive puts the device well intoits linear region of operation. Accordingly, saturation can only occurwhen the device is biased with its gate potential near threshold duringa transient.

FIG. 8 illustrates an experimentally measured body current (I_(body)) inMOSFET M1 as a function of gate drive (V_(gs)) for various drain biases(V_(ds)). For gate potentials above 1.4 V, the device comes out ofsaturation, the electric fields in the device drop and the body currentdrops precipitously. When V_(gs) is near zero, the body current is loweven though the fields are high. In between these regions is a peak inimpact ionization and therefore body current, since conduction currentis flowing through regions of high electric field. The effect of impactionization induced substrate current is discussed in S. M. Sze, Physicsof Semiconductor Devices, 2nd Ed., Wiley (1981), pp. 482-486. Themaximum body current occurs when V_(gs)≈1.3 V. When V_(ds) is equal to 8V, the maximum body current is around 3 mA. If the device were toconduct current with a V_(ds) of 16 V, the ionization current increasesto several hundred milliamps. These currents are, of course, transientin nature. (The device tested was the V30042 manufactured by Siliconixincorporated.)

FIG. 9 illustrates the relationship between the gate width of MOSFET M2or M3 and the body current I_(body) of MOSFET M1. V_(ds) of the MOSFETwas 0.3 V and V_(gs) was 8.4 V. FIG. 9 shows that the larger the gatewidth of the MOSFET, the higher the current it can sink withoutdeveloping any appreciable voltage drop. Using a design criterion ofsinking a current transient of at least 100 mA without generating aforward-bias of more than 0.3 V on the source-to-body diode of MOSFETM1, a gate width of at least 10 Kμm is prescribed. Safe operation wasconfirmed experimentally under these conditions using a device having agate width of 11,700 μm.

MOSFETs M2 and M3 also need to prevent forward-biasing of either of theintrinsic diodes in MOSFET M1 when MOSFET M1 approaches the so-calledsustaining voltage of its parasitic bipolar transistor. This sustainingvoltage of MOSFET M1 is shown in FIG. 10, is again using a V30042. InFIG. 10, the horizontal axis represents the drain-to-source voltageV_(ds) of MOSFET M1 and the vertical axis represents the drain currentI_(d) of MOSFET M1. The respective curves represent I_(d) as a functionof V_(ds) as the gate drive V_(gs) varies in increments of 50 mV from1050 mV to 1450 mV.

When the drain-to-source voltage V_(ds) of MOSFET M1 drops below thethreshold voltages of MOSFETs M2 and M3, the latter turn off and allowthe body of MOSFET M1 to float. FIG. 11 illustrates the current-voltagecharacteristics of one of MOSFETs M2 and M3 as a function of gate drive,as the gate drive (V_(gs)) varies from 0 V (far right curve) to 2 V. ForV_(gs)<1.4 V, several hundred millivolts may develop across one of theintrinsic diodes of MOSFET M1. When MOSFET M1 is fully turned on, itstotal V_(ds) may be only 100 mV, in which case the gate drive V_(gs) ofMOSFET M2 or M3 is also close to zero, and the body of MOSFET M1 couldfloat to a full diode voltage of 640 V, as shown in the far right curvein FIG. 11. If no body current were flowing in MOSFET M1, the lack of agate drive on MOSFET M2 or M3 would not be a problem, because even asmall amount of gate enhancement would turn on MOSFET M2 or M3 slightly,and force the body voltage of MOSFET M1 to the most negative potential.

However, since the IC represented by current source 51 (FIG. 5) alsogenerates current, and some of this current is necessarily carried bythe body of MOSFET M1, the body voltage of MOSFET M1 will tend to riseabove the lower of its source and drain voltages. If enough current isforced into the body of MOSFET M1 in its on condition, it is conceivablethat the voltage of the body could rise above both source and drain,forward-biasing both junctions. In fact, any time the voltage acrossswitch S6 becomes small, MOSFETs M2 and M3 cannot by themselves keep theintrinsic diodes in MOSFET M1 from becoming forward-biased. Theresulting drain current I_(d) in MOSFET M1 should be small, since the βof the parasitic bipolar transistor is low. The observed current is muchhigher, however, due to channel conduction in MOSFET M1.

This is illustrated in FIG. 12, wherein current source 51 againrepresents the body current I_(body) forced into MOSFET M1 by anexternal source. Assume that the gate of MOSFET M1 is tied to its bodyand the device is turned off. Sinking a current into the body terminalresults in a slight forward-biasing of the body-to-drain diode with avoltage V_(bd). The value of V_(bd) is determined by the followingequation: $V_{bd} = {\frac{kT}{q}\ln \frac{I_{body}}{I_{o}}}$

wherein K is Boltzmann's constant, T is temperature in ° C., q is thecharge of an electron and I_(o) is the saturation current of a diode.

Because the gate of MOSFET M1 is tied to its body, the gate is alsopulled to a more positive potential. This slight gate enhancement ofMOSFET M1 is then amplified by the large transconductance of the device.Because the V_(gs) of MOSFET M1 is small, the device is saturated. Evenignoring subthreshold conduction, the resulting current is at least:$I_{d} = {\frac{\mu \quad {CoxW}}{2L}\left\{ {{\frac{kT}{q}\ln \frac{I_{body}}{I_{o}}} - \left\lbrack {V_{to} - {\Delta \quad {V_{t}\left( V_{bd} \right)}}} \right\rbrack} \right\}^{2}}$

Wherein I_(d) represents the leakage current through MOSFET M1, μ is thechannel mobility, Cox is the gate capacitance, W is the channel width, Lis the channel length, V_(to) is the threshold voltage V_(t) absent thebody effect and Δ V_(t) is the shift in V_(t) caused by the body effect.Because the gate width W of MOSFET M1 is large, a high leakage currentI_(d) can result. FIG. 13 shows the current-voltage characteristics ofMOSFET M1 in the off condition (V_(gs)=0), as a function of bodycurrent. (MOSFET M1 was connected as shown in FIG. 12, without MOSFETsM2-M5.) As FIG. 13 indicates, a body current of 333 μA produces an I_(d)of 500 mA, resulting in a current gain of 170,000. For 3 mA of bodycurrent, MOSFET M1 doesn't even saturate at an I_(d) of 1 A.

FIG. 14 shows the same data when MOSFETs M2 and M3 (but not MOSFETs M4or M5) are connected. The maximum g I_(d) for 333 μA of body current isnow reduced to 100 mA. More importantly, this current can only beconducted up to a voltage of about 0.16 V, above which MOSFET M2 turnson and I_(d) collapses to zero. The curve for I_(body)=32 μA is notclearly visible in. FIG. 14, but is visible in the more detailed view ofFIG. 15. For a body current of 32 μA, the peak I_(d) is only 1.6 mA andMOSFET M1 begins to turn off at voltages over 80 mV. (Since these curveswere generated by a curve tracer, the double line in the curves resultsfrom the capacitance of the body.)

Despite these favorable results, it is still desirable to limit evenfurther the ability of the body of MOSFET M1 to float upward whenMOSFETs M2 and M3 are both turned off, thereby limiting the leakingcurrent I_(d) in MOSFET M1 even more. Such is the function of MOSFETs M4and MS.

FIG. 16 is a schematic diagram of MOSFET M4 or M5 as connected in thecircuit shown in FIG. 6. Since the gate, drain and body of the MOSFETsare shorted together, the MOSFET operates in a manner somewhat similarto a Schottky diode. This is because, when the source-to-body diode inthe MOSFET is forward-biased, the voltage barrier at the junctionbetween the source diffusion and the body diffusion is lowered, and thethreshold voltage is decreased. This is sometimes referred to as the“anti-body effect”, and it occurs in every MOSFET when it is operated inQuadrant 3 (negative V_(ds) and I_(d)). The anti-body effect is normallynegligible because it is very small relative to the threshold voltage orthe enhancement (gate drive) voltage. When the gate, drain and body of aMOSFET having a large channel width are shorted together, the voltagedrop across the resulting two-terminal device is lower than that of aconventional P-N diode. This voltage drop V_(ds), which is lower thanV_(to), can be expressed as follows:

V _(ds) =V _(to) +γ{{square root over (2φ_(f) −V _(sb))}−{square rootover (2φ_(f))}}≈V_(sb)

wherein γ is the body effect factor, φ_(f) is the Fermi potential(depends logarithmically on the doping N_(A) of the body, i.e.,φ_(f)=0.026 ln where n_(i) is the intrinsic carrier concentration ofsilicon), and V_(sb) is the source-to-body voltage. FIG. 17 is acurrent-voltage diagram which compares the characteristics of MOSFETs M4and M5 with a conventional P-N diode. It indicates that the voltage dropacross MOSFETs M4 or M5 when they are forward-biased is approximately220 mV less than the voltage drop across a forward-biased junctiondiode. The actual conduction in the MOSFET is the channel current, notminority carrier current, so the MOSFET is also very fast. Moreover,MOSFETs M4 and M5 are fully integratable.

FIG. 18 is a semi-log diagram comparing the current-voltagecharacteristics of a normal P-N diode, two embodiments of MOSFET M4 orM5, and a Schottky diode.

In some embodiments a Schottky or other type of diode could besubstituted for each of MOSFETs M4 and M5.

When combined with MOSFETs M2 and M3, MOSFETs M4 and M5 limit themaximum body voltage of MOSFET M1 to about 420 mV. FIG. 19 illustratesthe current-voltage characteristics of the parallel combinations ofMOSFETs M2/M4 and M3/M5 as a function of the gate drive voltage onMOSFETs M2 and M3. As shown, the voltage applied by the parallelcombination of MOSFETs drops below the voltage applied by MOSFETs M4 antM5 alone as the gate drive voltage on MOSFETs M2 and M3 increases. Asshown in FIG. 20, with MOSFET M1 turned off the combination of MOSFETsM4 and M5 with MOSFETs M2 and M3 together limits the maximum draincurrent I_(d) in MOSFET M1 to about 30 μA for a 30 μA body current.Thus, the net effect is a peak MOSFET parasitic current gain of only 1.The leakage current I_(d) shuts off beyond a V_(ds) in MOSFET M1 of 150mV.

A family of current-voltage curves for MOSFET M1 is shown in FIG. 21.The four curves are for an I_(body) of 20 μA, 40 μA, 60 μA and 80 μA.(The curves are actually symmetrical about the point of origin; theoffset on the vertical (I_(d)) axis in Quadrant III is an artifact ofthe current sense circuit of the curve tracer, which is referenced toground. In Quadrant I (positive drain voltage and current), the basebias generator current does not flow through the sense resistor and doesnot appear in the curves. In Quadrant III (negative drain voltage andcurrent), the current through the base bias generator (which is stillmore positive than ground) flows through the sense resistor, causing aconstant offset in drain current for each of the curves.)

Like MOSFETs M2 and M3, the size of MOSFETs M4 and M5 can be adjusted toaccommodate more current. The data shown previously assume a gate widthof 10,000 μm for MOSFETs M4 and M5. The simulated data in FIG. 22 showthat this gate width corresponds to a voltage drop of about 400 mV. Thedrain current I_(d) was 30 μA. Some is oversizing of MOSFETs M4 and M5may be desirable to accommodate for manufacturing variations in thethreshold voltage.

The net current-voltage characteristics of battery disconnect switch S6,including MOSFETs M2 and M3 and MOSFETs M4 and M5, is shown in FIG. 23.Note the inflection in the lower V_(gs) curves at about 150 mV.

When a battery charger is connected in reverse, as shown in FIG. 4F, thetotal voltage across the battery disconnect switch may reachapproximately 16.8 V (assuming an 8.4 V battery). This voltage typicallyexceeds the maximum voltage allowed on the gate of MOSFET M2. While avoltage of 16.8 V will not rupture the gate oxide of MOSFET M2, forlong-term reliability of the device it is desirable to clamp this gatevoltage at around 15 V (assuming a gate oxide thickness of 400 Å). FIGS.24A and 24B illustrate two arrangements for accomplishing this.

In FIG. 24A a zener diode D1 is connected between the body of MOSFET M1and the gate of MOSFET M2, and a current-limiting resistor R1 isconnected between the source of MOSFET M1 and the gate of MOSFET M2. Ifit is desired to clamp the gate voltage of MOSFET M2 at 15 V, zenerdiode D1 should have a breakdown voltage of 15 V. Zener diode can easilybe integrated into a discrete battery disconnect switch withoutrequiring any additional external connections. The current at breakdownof diode D1 can be limited to the μA level by making resistor R1 a highvalue (small area) drift resistor (i.e., the N-drift implant used in theformation of the symmetrically drifted MOSFET M1 may be used as a highsheet resistance small area resistor). The only time that diode D1 willexperience avalanche breakdown is during a reverse charger connectionwhere the sum of the charger and battery voltages exceed its breakdownvoltage. Otherwise, diode D1 remains off and actually provides somedegree of ESD protection to the gate of MOSFET M1.

In FIG. 24B, a cascode N-channel MOSFET M6 is connected into thecircuit. MOSFET M6 is a four-terminal device with no source/body short.The source/drain terminals of MOSFET M6 are connected to the source ofMOSFET M1 and gate of MOSFET M2, respectively; the body of MOSFET M6 isconnected to the body of MOSFET M1; and the gate of MOSFET M6 isconnected to the positive terminal of battery B, a fact which makes itadvantageous to implement body bias generator 50 inside the control IC.FIG. 24C shows a similar circuit in which MOSFET M6 is connected at adifferent position in the current path from the positive terminal ofbattery charger Y to the gate of MOSFET M2. This arrangement has theadvantage that the resistance of MOSFET M6 does not appear in thecurrent path through MOSFETs M4 and M5 in parallel with the conductionpath through MOSFET M1.

FIG. 25A shows a cross-sectional view of an integrated circuitembodiment of the circuit shown in FIG. 24A. Each of MOSFETs M1-M5 andzener diode D1 is shown schematically above the corresponding device ina P-epitaxial layer 100, which is formed on a P+ substrate 101. Asindicated, MOSFET M1 is a four-terminal device having no source/bodyshort, while MOSFETs M2-M5 each contain a source/body short. Field oxideregions separate MOSFET M1, MOSFETs M2 and M3, MOSFETs M4 and M5 anddiode D1, respectively. As shown by the dashed line, the connection ofdiode D1 into the circuit is optional. MOSFETs M4 and M5 have thresholdadjustment implants in accordance with the teachings of theabove-referenced application Ser. No. 08/160,539.

MOSFETs M4 and M5 may optionally be formed in a P-well having a dopantconcentration higher than that of P-epitaxial layer 100. The higherconcentration of P-type ions raises the body factor γ of MOSFETs M4 andM5, without substantially increasing their threshold voltage, andtherefore causes MOSFETs M4 and M5 to turn on more quickly as thevoltage at the body of MOSFET M1 increases (in a negative direction).This is desirable in order to prevent the intrinsic diodes in MOSFET M1from becoming forward-biased. This is apparent from FIG. 26, whichillustrates a graph showing the threshold voltage V_(t) of a MOSFET as afunction of the source-to-body voltage V_(sb) at two levels of the bodyfactor γ. In Quadrant I, where MOSFET M1 operates, a lower γ provides alower V_(t) at a given V_(sb). In Quadrant III, where MOSFETs M4 and M5operate, a higher γ provides a lower V_(t) at a given V_(sb).

FIG. 25B is a similar drawing showing the circuit of FIG. 24C inintegrated circuit form.

MOSFETs M1 and MOSFETs M2-M5 (body bias generator 50) may be formed as adiscrete device separate from the gate control circuitry (e.g.,comparator S, OR gate T and inverter U shown in FIG. 7C), which may beformed on an IC; or body bias generator may be formed on the IC with thegate control circuitry and MOSFET M1 may be a discrete element.

In the alternative embodiment shown in FIG. 27, a second body biasgenerator 50A, matched to body bias generator 50, is included in anintegrated circuit Z along with the gate control circuitry. The outputof body bias generator 50A is used to drive the gate of MOSFET M1 viainverter U. Since body bias generator 50 and 50A are matched, a“virtual” connection is established between the gate and body of MOSFETM1 when MOSFET M1 is turned off. Body bias generator 50A also preventsdiodes within integrated circuit Z from becoming forward biased. Bodybias generator 50 could also be included in integrated circuit Z.

While specific embodiments of this invention have been described, it isunderstood that these embodiments are illustrative and not limiting. Thebroad scope of this invention is limited only by the following claims.

We claim:
 1. An arrangement comprising: a battery; a first MOSFET havinga drain terminal, a source terminal, a body and a first gate, said drainterminal being connected to a negative terminal of said battery, saidfirst MOSFET further comprising a first intrinsic diode between saidbody and said source terminal and a second intrinsic diode between saidbody and said drain terminal; a body bias generator connected to saiddrain terminal and said source terminal of said first MOSFET, said bodybias generator biasing said body of said first MOSFET to the lower of adrain voltage at said drain terminal and a source voltage at said sourceterminal when a difference between said drain and source voltagesexceeds a predetermined level, said body bias generator comprising: asecond MOSFET connected between said body and said source terminal, saidsecond MOSFET having a second gate connected to said drain terminal; anda third MOSFET connected between said body and said drain terminal, saidthird MOSFET having a third gate connected to said source terminal; anda gate control circuit, said gate control circuit alternately biasingsaid first gate to a voltage at said body so as to turn said firstMOSFET off or to a positive voltage so as to turn said first MOSFET on,wherein said second MOSFET is adapted to turn on and conduct a firstcurrent between said body and said source terminal at a voltage dropacross said second MOSFET which is less than a first voltage droprequired to turn on said first intrinsic diode and said third MOSFET isadapted to turn on and conduct a second current between said body andsaid drain terminal at a voltage drop across said third MOSFET which isless than a second voltage drop required to turn on said secondintrinsic diode, further comprising a second body bias generator havingan output terminal connected to said gate control circuit, said secondbody bias generator being connected to said drain terminal and saidsource terminal of said first MOSFET, said second body bias generatorproviding at said output terminal an output voltage which is equal tothe lower of a drain voltage at said drain terminal and a source voltageat said source terminal when the difference between said drain andsource voltages exceeds a predetermined level, said body bias generatorand said second body bias generator thereby forming a virtual connectionbetween said body and said first gate of said first MOSFET when saidfirst MOSFET is turned off.
 2. The arrangement of claim 1 wherein saidpositive voltage is equal to a voltage at a positive terminal of saidbattery.
 3. The arrangement of claim 1 wherein said second MOSFET has athreshold voltage the absolute value of which is less than said firstvoltage drop and said third MOSFET has a threshold voltage the absolutevalue of which is less than said second voltage drop.
 4. An arrangementcomprising: a battery; a first MOSFET having a drain terminal, a sourceterminal, a body and a first gate, said drain terminal being connectedto a negative terminal of said battery; a body bias generator connectedto said drain terminal and said source terminal of said first MOSFET,said body bias generator biasing said body of said first MOSFET to thelower of a drain voltage at said drain terminal and a source voltage atsaid source terminal when a difference between said source and drainvoltages exceeds a predetermined level, said body bias generatorcomprising: a second MOSFET connected between said body and said sourceterminal, said second MOSFET having a second gate connected to saiddrain terminal; and a third MOSFET connected between said body and saiddrain terminal, said third MOSFET having a third gate connected to saidsource terminal; and a gate control circuit, said gate control circuitalternately biasing said first gate to a voltage at said body so as toturn said first MOSFET off or to a positive voltage so as to turn saidfirst MOSFET on, wherein a peak parasitic current gain (I_(d)/I_(body))in said first MOSFET when said first MOSFET is turned off is greaterthan zero and less than or equal to
 300. 5. An arrangement comprising: abattery; a first MOSFET having a drain terminal, a source terminal, abody and a first gate, said drain terminal being connected to a negativeterminal of said battery; a body bias generator connected to said drainterminal and said source terminal of said first MOSFET, said body biasgenerator biasing said body of said first MOSFET to the lower of a drainvoltage at said drain terminal and a source voltage at said sourceterminal when a difference between said source and drain voltagesexceeds a predetermined level, said body bias generator comprising: asecond MOSFET connected between said body and said source terminal, saidsecond MOSFET having a second gate connected to said drain terminal; anda third MOSFET connected between said body and said drain terminal, saidthird MOSFET having a third gate connected to said source terminal; afourth MOSFET connected in parallel with said second MOSFET; and a fifthMOSFET connected in parallel with said third MOSFET; and a gate controlcircuit, said gate control circuit alternately biasing said first gateto a voltage at said body so as to turn said first MOSFET off or to apositive voltage so as to turn said first MOSFET on, wherein a peakparasitic current gain (I_(d)/I_(body)) in said first MOSFET when saidfirst MOSFET is turned off is greater than zero and less than or equalto one.
 6. An arrangement comprising: a battery; a first MOSFET having adrain terminal, a source terminal, a body and a first gate, said drainterminal being connected to a negative terminal of said battery; a bodybias generator connected to said drain terminal and said source terminalof said first MOSFET, said body bias generator biasing said body of saidfirst MOSFET to the lower of a drain voltage at said drain terminal anda source voltage at said source terminal when a difference between saidsource and drain voltages exceeds a predetermined level, said body biasgenerator comprising: a second MOSFET connected between said body andsaid source terminal, said second MOSFET having a second gate connectedto said drain terminal; and a third MOSFET connected between said bodyand said drain terminal, said third MOSFET having a third gate connectedto said source terminal; a fourth MOSFET connected in parallel with saidsecond MOSFET; and a fifth MOSFET connected in parallel with said thirdMOSFET; and a gate control circuit, said gate control circuitalternately biasing said first gate to a voltage at said body so as toturn said first MOSFET off or to a positive voltage so as to turn saidfirst MOSFET on, wherein a maximum drain current in said first MOSFET assaid difference between said source and drain voltages is increased fromzero while said first MOSFET is turned off is greater than zero and lessthan or equal to 30 μA.
 7. An arrangement comprising: a battery; a firstMOSFET having a drain terminal, a source terminal, a body and a firstgate, said drain terminal being connected to a negative terminal of saidbattery; a body bias generator connected to said drain terminal and saidsource terminal of said first MOSFET, said body bias generator biasingsaid body of said first MOSFET to the lower of a drain voltage at saiddrain terminal and a source voltage at said source terminal when adifference between said drain and source voltages exceeds apredetermined level, said body bias generator comprising: a secondMOSFET connected between said body and said source terminal, said secondMOSFET having a second gate connected to said drain terminal; and athird MOSFET connected between said body and said drain terminal, saidthird MOSFET having a third gate connected to said source terminal; anda gate control circuit, said gate control circuit alternately biasingsaid first gate to a voltage at said body so as to turn said firstMOSFET off or to a positive voltage so as to turn said first MOSFET on,wherein said second MOSFET is adapted to turn on and conduct a firstcurrent between said body and said source terminal at a voltagedifference across said second MOSFET which is less than 640 mV and saidthird MOSFET is adapted to turn on and conduct a second current betweensaid body and said drain terminal at a voltage difference across saidthird MOSFET which is less than 640 mV.
 8. The arrangement of claim 4wherein said peak parasitic current gain (I_(d)/I_(body)) in said firstMOSFET when said first MOSFET is turned off is greater than zero andless than or equal to 192.